This invention pertains to horn gaps, a type of lightning arrester used to protect ungrounded antenna systems. More specifically, this invention pertains to an improved mechanism for adjusting the width of the gap between the electrodes of this device without requiring that the antenna's radiating power be shut off.
Horn gaps have been used as lightning arresters since early in the twentieth century. They typically contain two bent metal rods separated by a gap that function as electrodes, and which can be described as roughly looking like a pair of animal horns. One rod, the cold horn, is grounded, while the other rod, the hot horn is not. The function of the horn gap is to flash over lightning energy to ground initially and then to dissipate the lightning energy to ground between the horns of the gap. It is used to protect the components and insulators of ungrounded antenna systems such as those commonly used as standard AM radio stations, "T" type antennas, curtain antennas or any other antenna system whose structure is required to be above ground potential.
Certain methods of modulation, especially amplitude modulation (AM) create substantial variation in power level. An amplitude modulated carrier varies in power level from 0 to 250 percent of carrier power. For a 50,000 watt power level carrier, the power varies instantaneously from 0 watts to 125,000 watts. It is difficult to obtain the proper gap width of a lightning arrester. If the gap is set too small, the gap will flash over upon normal modulation. If the gap is set too wide, the gap will not flash over soon enough and lightning damage will occur. A balance must be obtained empirically between a gap width that will not flash over under normal power level variations from modulation, but will flash over in the presence of lightning. A proper gap width may range between 1.0 cm and 2.5 cm. A lightning arrester, therefore, has means to adjust the gap width. In a conventional horn gap lightning arrester, such as taught by Jacobs in U.S. Pat. No. 1,755,324, a portion of a horn can be slid in a bracket. When an appropriate gap width is reached a set screw can be tightened to secure the horn's position. In such a design, the position of the horn cannot be changed while the antenna is radiating power because the mechanism for changing the gap width is at above ground potential and there risk of severe electrical shock, burns, or even electrocution. In conventional designs, the radiating power is turned off, an adjustment is made to the hot horn and then radiating power is reapplied. This procedure is repeated until it is believed a correct setting is achieved. This setting cannot be changed until the operation can be shutdown again for readjustment. Thus the object of the instant invention to provide a mechanism for adjusting the width of the gap in a lightning arrestor that can be used when the antenna is radiating power. Being able to adjust the width while the antenna is radiating power facilitates finding a proper setting because the flashing will occur under normal power variations when the gap width is too small.